The present invention relates to heap leach mining. More specifically, the present invention relates to a method and apparatus used in heap leach mining that involves solar heating and distribution of a leach solution prior to its percolation through an ore heap.
Copper is frequently recovered from extremely low grade ores and from mine wastes by a process known as heap leaching. The term "heap" as used in the art means a bed of low grade ore that has been suitably spread over a prepared surface or "pad". A dilute sulfuric acid solution is percolated through the heap and the "pregnant" copper-bearing acid solution is collected after it has percolated through the heap. Typically, the pad on which the heap rests is formed from an impervious material, such as sheets of plastic film (polyethylene), asphalt and/or compacted clay. The heap is often times constructed on an incline such that the copper-bearing leach solution that has percolated through the heap is collected in a reservoir.
The heap of ore is normally piled onto each impervious pad to a depth of 10 to 30 feet after the ore has been pre-crushed to a sufficiently small size to enable the leach solution to reach the metallic-mineral particles contained in the ore. It is a characteristic of heap leach mining that the maximum amount of copper can be recovered from the ore over a long period of time, typically measured in months and years. For example, a typical heap leach operation may reach a maximum copper recovery in a period of two to three years. After the leach solution has passed through the ore heap, the copper-bearing leach solution is subjected to a solvent extraction process in which the copper is transformed from dilute leach solution into a pure and concentrated solution termed advance electrolyte. After the extraction, the advance electrolyte is subjected to an electrowinning process in which the dissolved copper plates out onto permanent stainless steel plates or pure thin copper sheets used as cathodes. The plated copper on each of the stainless steel cathodes can be removed and processed. The copper sheets, when used, grow in weight and can be processed as required.
Conventional percolation systems for heap leach mining normally includes a series of individually spaced emitter tubes extending over the heap of ore. Each of the spaced tubes receives the leach solution and includes a series of openings to permit the leach solution to be distributed for uniform percolation down through the heap. An example of such a system is shown in the Krauth U.S. Pat. No. 5,030,279. In this type of system, each of the individual tubes must be laid out over the ore heap in the desired spaced relationship such that the leach solution adequately permeates through the ore heap. Thus, a great deal of care and time is required to accurately lay out the individual tubes required to adequately distribute the leach solution over the heap.
In heap leach operations, it has been found that metal recoveries are considerably lower in the winter season due to lower temperatures of the ore and the leach solution. Heap leach operations, utilizing both chemical and biological leach solutions, tend to operate most efficiently at temperatures above ambient. While others have proposed heating the leach solution prior to its application to the heap, heating the leach solution has proved to be very inefficient due to the substantial amount of heat lost to atmosphere as the leach solution is distributed through the tubing placed on the heap.
Therefore, it is an object of the present invention to provide a method of increasing the rate at which a semi-precious metal can be extracted from an ore heap. It is a further object of the invention to provide a method that solar heats a leach solution as the leach solution is distributed over the bed of ore. It is an additional object of the invention to provide a method that solar heats the leach solution immediately prior to the leach solution being percolated through the ore heap. Further, it is an object of the invention to provide an apparatus that increases the temperature of the leach solution immediately prior to the leach solution being applied to the ore heap. It is a further object of the invention to provide an apparatus that solar heats the leach solution prior to its percolation through the ore heap. It is an additional object of the invention to provide an apparatus including a series of emitter tubes joined by a heat absorbing panels to form a flexible distribution mat, such that the heat absorbing panels of the distribution mat form a solar collector to heat the leach solution as it is applied to the ore heap. Further, it is an object of the invention to provide an apparatus that can be positioned over the top of an ore heap that joins the individual emitter tubes to provide the required spacing for the tubes.